Manufacturers of electronic components use a variety of processing techniques to fabricate semiconductor devices. One technique that has many applications (e.g. deposition, etching, cleaning, annealing) is known as "plasma-assisted" processing which is a dry processing technique. In plasma-assisted processing, a substantially ionized gas, usually produced by high-frequency electrical discharge, generates activated metastable neutral and ionic species that chemically react to deposit thin material layers or to etch materials on semiconductor substrates in a processing reactor.
Various applications for plasma-assisted processing in semiconductor device manufacturing may include high-rate reactive-ion etching (REI) of thin films of polysilicon, metals, oxides, and polyimides; dry development of exposed and silylated photoresist layers; plasma-enhanced chemical-vapor deposition (PECVD) of dielectrics, aluminum, copper, and other materials; low-temperature dielectric chemical-vapor deposition for materials such as boron nitride and silicon nitride; planarized interlevel dielectric formation, including procedures such as bias sputtering; and low-temperature epitaxial semiconductor growth processes.
Reactive ion etching (RIE), an example of low-pressure plasma-assisted processes, uses the directional and energetic ions in the plasma to selectively etch materials on a semiconductor substrate that is directly proximate to the plasma environment. RIE processes can take place in the conventional parallel-plate radio-frequency (RF) powered plasma processing reactors or similar semiconductor device fabrication equipment. In RIE, a high-power (100-1000 W) radio-frequency (RF) electrical power source interacts with process gases to produce a plasma that can interact with the semiconductor wafer. The frequency of the RF power source is usually in the range of 100 KHz-30 MHz (mostly 13.56 MHz).
In conventional plasma processes such as RIE, there is a trade-off between processing rate or throughput and the semiconductor device quality. To enhance the RIE processing rate, the plasma density and ion flux should be increased. According to conventional plasma processing methods, increasing the RF power that produces the plasma via electromagnetic gas discharge increases ion density. Increasing the RF power to the plasma medium, however, also raises the average plasma ion energy levels, and ions with excessive directional energies (e.g. several hundred electron volts) may damage the semiconductor devices. This is because the ions are so energetic that upon impact they penetrate and cause irradiation damage to the semiconductor device surface. When this type of ion-induced radiation damage occurs, a post-fabrication cleansing or annealing process is necessary to minimize the adverse effects to semiconductor device performance. Moreover, many anisotropic plasma etch processes based on RIE usually leave undesirable chemical deposits such as fluorohydrocarbons on the semiconductor wafer surface, resulting in manufacturing yield degradation. Ultimately, the manufacturer must remove these deposits from the semiconductor surface by some post-etch cleaning. In conventional plasma processing techniques, the plasma medium can interact with the plasma chamber walls, resulting in deposition of various contaminants (such as metals) onto the semiconductor wafer (contaminants are sputter etched from the plasma electrodes and reactor walls).
The combined effects of irradiation damage, formation of fluoro-carbon films, plasma-induced contaminants, and other undesirable phenomena produce semiconductor devices with less than optimal performance characteristics and limit device fabrication process yield. Thus, with conventional plasma-assisted processing techniques, increasing RF power to increase ion density with the intent to raise the process rate can have serious detrimental effects. If a method existed, however, to increase the plasma density and ion flux without also increasing ion energies, then a manufacturer may increase plasma-assisted processing rates without device yield degradation.
Therefore, a need exists for a method and apparatus to increase ion density near a semiconductor device during plasma-assisted processing without at the same time increasing the average ion energy levels.
As mentioned earlier, another limitation of conventional plasma-assisted processes derives from the fact that, during these processes, plasma disperses throughout the fabrication process chamber. In so doing, it interacts with the process chamber walls. These walls contain various metals that the plasma species can remove via sputter etch or chemical reactions, transport to the semiconductor device surface, and embed into the semiconductor device. As a result, further semiconductor device degradation occurs.
Consequently, there is a need for a method and apparatus to prevent plasma interaction with fabrication reactor process chamber walls during plasma-assisted processing.
To remedy the above problems in plasma-assisted processing, manufacturers often use techniques known as "magnetron-plasma-enhanced" (MPE) processing. MPE processing basically entails applying a predetermined magnetic field in the proximity of the semiconductor wafer during the plasma-assisted process. During an MPE process, the magnetic field confines the plasma and causes the plasma to appear as a gaseous ball enveloping the semiconductor wafer and centered therewith. As a result, the plasma ion density is greatest near the semiconductor wafer, and the plasma that the semiconductor substrate sees does not interact significantly with the process chamber walls. This plasma magnetization and confinement provides improved gas excitation and higher plasma density than with conventional plasma processing techniques. MPE processing raises the device processing rate, increases equipment throughput, and reduces semiconductor device degradation from process chamber wall contaminants by making the plasma concentrate near the semiconductor substrate. Thus, MPE processing can produce higher semiconductor device processing rates without an increase in local plasma ion energies and without ion-induced irradiation damage.
Conventional MPE apparatus designs employ two permanent magnet bars of opposite polarities positioned near the semiconductor wafer in the fabrication reactor. These permanent magnets produce a magnetic field near the semiconductor device surface which has both transverse and longitudinal magnetic flux components. The magnitudes of these magnetic field components may vary significantly over the wafer surface. These magnetic field non-uniformities cause plasma ion density non-uniformities over the semiconductor device. Consequently, a semiconductor device layer deposition or etching resulting from these conventional MPE processes will also suffer from non-uniformities. Moreover, the conventional magnetron plasma modules do not provide any capability for magnetic field uniformity adjustment. As a result, process uniformity optimizations are performed by adjusting other process parameters such as RF power and process pressure. Complete uniformity optimization via other process parameters (e.g., RF power) is often impossible or very difficult.
The transverse and longitudinal magnetic flux components from conventional magnetron plasma modules in a typical cylindrical processing chamber can produce both radial and tangential process non-uniformities. By rotating the conventional MPE magnetron in the horizontal plane, a conventional MPE magnetron may produce a more uniform magnetron plasma process with some cylindrical symmetry. Rotating conventional MPE magnetrons in this manner requires a more complex fabrication reactor, because the magnetron module components are usually embedded within the RF plasma chuck against which the semiconductor wafer is clamped.
Thus, there is a need for a method and apparatus that provides a suitable magnetic field distribution for MPE processing of semiconductor wafers of various sizes (e.g. 6"-10" in diameter) without the process non-uniformity problems of conventional magnetron plasma modules.
While conventional magnetron plasma modules may be able to produce a somewhat uniform transverse magnetic field because of the magnetic flux field shape they produce, they cannot usually produce a uniform MPE etch or deposition process. Moreover, the conventional magnetron plasma modules are not usually easily scalable for larger wafer processing. To overcome the limitations of the conventional MPE processing magnetrons, one approach is to adjust the RF power extracted from the RF source. This adjustment, however, can adversely affect other parameters associated with RF power. This is because, as already stated, varying the RF power affects the plasma ion energy levels as well as the plasma density.
As a result, there is a need for a magnetron plasma processing module for producing a suitable magnetic field distribution for uniform MPE processing of semiconductor wafers of various sizes.